1. Field of the Invention
This invention relates to a method and apparatus for low NO.sub.x, staged-air combustion within a combustion chamber in which the flame from said combustion is controlled in a manner which substantially reduces the entrainment of secondary combustion air into the root of the flame.
2. Description of Prior Art
The reduction of NO.sub.x emissions from combustion processes and apparatus is a long standing, major environmental objective. As a result, numerous strategies for reducing the NO.sub.x emissions from combustion processes have been developed. One such strategy is the use of staged combustion in which the fuel or combustion air is introduced into the combustion apparatus in two or more stages thereby maintaining flame temperature below a threshold temperature required for significant formation of NO.sub.x in the combustion process. U.S. Pat. No. 4,021,186, for example, teaches a forced draft burner operating at below stoichiometric mixtures of air and fuel in a primary combustion chamber and completion of the combustion by injection of secondary air near the outlet of the combustion chamber to reduce NO.sub.x produced by combustion of nitrogen-containing fuels.
U.S. Pat. No. 4,403,941 teaches a combustion process for reducing nitrogen oxides in which combustion occurs successively in at least three combustion zones, an incomplete combustion zone, a reducing combustion zone, and a complete combustion zone, respectively, said combustion zones corresponding to primary burners, secondary burners and air ports or after-burners, successively arranged in the direction of the resulting gas stream in a combustion chamber.
Two-stage combustion of nitrogen-containing fuel with low NO.sub.x emissions in a combustion chamber is also taught by U.S. Pat. No. 5,002,483 wherein first, precombustion is performed in a primary stage under fuel-rich conditions and, subsequent, post combustion is performed in a secondary stage under fuel-lean conditions and an inert substance is supplied to the primary stage to cool the walls of the combustion chamber and maintain the combustion zone remote from the walls. See also related U.S. Pat. No. 5,052,919.
U.S. Pat. No. 5,139,414 teaches a burner having opposed nozzles positioned adjacent an exit opening of a fuel supply passage defined by a combustion chamber wall and secondary air supply ports defined by the combustion chamber wall adjacent the exit of the combustion chamber to provide a primary combustion zone in which a substantial portion of the combustion occurs and a secondary combustion zone in which unburned components from the primary combustion zone are substantially completely burned.
Finally, U.S. Pat. No. 4,054,028 teaches a fuel combustion apparatus having a double-shell construction, a fuel injection valve and first air supply port equipped with swirl blades surrounding the fuel injection valve at the upstream end of the apparatus and secondary air supply ports in the downstream end of the combustion apparatus.
In addition to the use of staged combustion, the use of flame inserts for reducing NO.sub.x emissions, particularly in atmospheric-type burners is generally known. U.S. Pat. No. 4,525,141 teaches an atmospheric-type burner having a flame insert which reduces the peak flame temperatures experienced in the combustion process by radiating heat away from the flame, thereby depressing the reaction kinetics associated with the fixation of atmospheric nitrogen. The flame insert includes flame guiding and cooling portions which include at least one insert surface for contacting the array of flames of the burner when the array assumes its normal combustion position in accordance with the flame directions of the individual flames or flamelets under the influence of their natural buoyancy during combustion. The flame insert consists of a pair of substantially circular ring insert elements of different diameters which are concentrically disposed about an array of ports of the atmospheric burner assembly. The insert elements include insert surfaces located adjacent the inner cones of the flame when the array of flames assumes its normal combustion position. The insert surfaces are spaced from one another with the inner cones of the flames extending between and engaging the insert surfaces during combustion. Related U.S. Pat. No. 4,629,415 teaches a flame insert for use with range top burners to regulate and reduce NO.sub.x emissions which includes a flame engaging portion and a support portion which mounts the insert to the burner and accurately positions the flame engaging portion adjacent to the burner ports.